Recent efforts towards developing smaller size electronic components have led to the development of chip carriers which are comprised of a planar housing (e.g., ceramic) containing a semiconductor chip therein. Generally, there are two types of chip carriers, those having wire-like leads extending therefrom and those having no leads at all. Chip carriers which have no leads are designated as being "leadless". Instead of having leads, leadless type chip carriers have metallized pads on a major surface of the housing connected to the semiconductor chip therein.
In practice, leadless type chip carriers are mounted on a printed circuit board by placing the pads thereof in contact with metallized areas on a major surface of the printed circuit board. The metallized areas on the circuit board are arranged in the same pattern as that of the pads. Once the pads are in contact with the metallized areas on the printed circuit board, the pads are typically solder-bonded thereto. To facilitate bonding of the pads on the chip carrier to the metallized areas on the surface of the circuit board, typically, the pads are each provided with a deposit or bump of solder thereon. When reflowed, the solder bumps establish a solid mechanical and electrical connection between the pads and the metallized areas.
Typically, a spherical solder preform is bonded to each pad to provide the solder bump thereon. During the process of bonding the solder preform to each pad, defects can occur. For instance, the solder preform may not adhere to the pad, so that the pad will have no solder bump thereon. Also, during the bonding of the solder preforms to the pads, the preforms on adjacent pads may merge. Thus, the resultant solder bumps on the pads become bridged together, giving rise to an electrical short circuit. A defect may also exist if the solder bump on a particular pad is excessive (too large). A solder bump which is excessive may serve to bond the corresponding pad to more than one corresponding metallized area on the surface of the printed circuit board which is not desirable. On the other hand, a solder bump which is very small, due to poor reflow during the bonding of the preform to the pad, may not create a good bond between the pad and the metallized area.
In the past, chip carriers have been inspected visually by a human operator to detect defects such as missing, shorting or excessive solder bumps. Manual inspection of chip carriers is time consuming and is prone to inaccurate results. Even the most experienced human operator will miss, on average, 25% of the defects in a batch of chip carriers. Often two or more human operators are employed to inspect the same batch of chip carriers in order to assure a high degree of reliability. Further, multiple operations are often required simply to achieve required levels of production throughput. Using multiple operators to perform chip carrier inspection greatly increases overall production costs which is undesirable. In an effort to avoid some of these difficulties, oftentimes only a small percentage of the chip carriers is actually inspected. Elimination of 100% inspection increases the chance that a chip carrier having some type of defect with regard to the solder bumps thereon may be mounted on a printed circuit board.
The task of visually examining the chip carrier for defects can be made easier by employing a video system of the type disclosed in U.S. Pat. No. 4,028,728 which issued June 7, 1977 to Benny H. Sharp and assigned to the instant assignee. The video system disclosed in the aforementioned patent comprises a polarized ring lamp which illuminates all sides of a planar article positioned therebeneath. A television camera, having polarized lens thereon, is positioned in registration with the ring lamp above the illuminated article. Only three-dimensional, light-reflective mesas, such as solder bumps, on the surface of the article serve to reflect the polarized light from ring lamp upwardly into the television camera. Thus, only the image of the solder bumps will be captured by the television camera. A video monitor is coupled to the television camera so that the operator can observe an enlarged image of the solder bumps on the video monitor without eye strain, thereby enabling more accurate article inspection.
The video system described in the aforementioned patent to Sharp may reduce the number of operators required to perform reliable article inspection, but will not eliminate the need for human operators entirely. A human operator is still required to analyze the image of the solder bumps displayed on the video monitor to determine if a defect is present.
Accordingly, there is a need for a technique for high speed, reliable automatic inspection of articles having light reflective mesas on a planar surface thereof to detect for defects.